Field of the Invention
The present invention relates to the removal of suspended solids from liquid streams and in particular to the removal of colloidal corrosion products from nuclear reactor coolant streams.
In the operation of power plants, a maintenance problem exists due to the presence of corrosion products which form in the plant cooling system and which are then deposited on the surfaces of the cooling system. The problem is particularly acute in nuclear power plants wherein deposited primary cooling system corrosion products are a source of radiation exposure to operating and maintenance personnel and contribute to the high cost of maintenance of such power plants. The primary cooling system corrosion products are generally solids which are produced principally by the corrosion of the steam generator tubes and to a lesser extent by corrosion of the other stainless steel plant surfaces.
The corrosion product components circulate with the primary coolant, both as a suspended or insoluble solid phase and to some extent as ions in solution. The solid phase particles deposit on the reactor core surface where they become irradiated. After being irradiated, the solid phase particles become resuspended in the primary coolant solution and are thereafter deposited on plant surfaces away from the core where the irradiated solid phase particles are the most important source of radiation exposure to personnel in the power plant.
In a pressurized water reactor (PWR) nuclear power plant, both the primary and secondary coolant streams carry a burden of insoluble magnetic corrosion products. These corrosion products also carry a surface charge, generally dependent on the pH of the coolant and, at least in the smaller particle sizes, are subject to electrostatic attraction which causes particles to adhere to the surfaces of the system.
The primary cooling system corrosion products are largely nickel ferrite and a nickel ferrite composition wherein cobalt, manganese and other elements have been substituted for part of the nickel. Substitution of chromium for ferric iron in the corrosion products has also been indicated. Such corrosion products have become colloquially known as crud.
It has been recognized in the literature that by far the major source (i.e., 70 to 90%) of occupational radiation exposure to the operators and maintenance personnel of nuclear power plants is the gamma radiation emanating from deposits of irradiated corrosion products which are ubiquitously distributed on the primary coolant system surfaces. In this connection, reference is made to a publication of Beslu et al. (P. Beslu and G. Frejaville, Occupational Radiation Exposure at French Power Plants: Measure and Prediction, Nuclear Technology, Vol. 44, pp. 84-90 (June, 1979)) and to a publication of Troy et al. (M. Troy, S. Kang, G. T. Zirps and D. W. Koch, Effect of High-Temperature Filtration on PWR Plant Radiation Fields in the book "Decontamination and Decommissioning of Nuclear Facilities", M. M. Osterhout editor, Plenum Press, New York, (1980)). Such corrosion products, or crud, are released from surfaces, transported as small particles and colloids to and deposited on the reactor fuel elements where they are activated by the neutron flux. The crud is then resuspended and transported as particles back to the other system surfaces. The chemistry of the reactor coolant and the electric charge of the surfaces, both particle and plant, determine to a large extent the rate of deposition. Other effects such as hydraulic forces and solution phenomena contribute to this distribution. The latter are believed to play only a minor role in the overall process, a situation which was discussed in a publication of Kang et al. (S. Kang, Y. Solomon and M. Troy, Reactor Coolant High-Temperature Filtration Volume 2: Evaluation of Effectiveness in Reducing Occupational Radiation Exposure, Electric Power Research Institute (EPRI) Report NP-3372, Volume 2, Research Project 1445-2 (May, 1984)). Accordingly, it is known that these insoluble corrosion products known as crud have an important adverse effect on personnel safety and plant availability and their removal carries a strong economic incentive.
The insoluble magnetic ferrite corrosion products carry a surface charge dependent on the pH of the coolant, see Moroto, et al. (A. J. G. Moroto, M. A. Blesa, S. I. Passagio, and A. E. Regazzoni, Colloidal Interactions on the Deposition of Magnetite Particles on the Fuel Elements Surface, Paper No. 36, Conference on Water Chemistry of Nuclear Reactor Systems, Bournemouth, England, (October, 1980)) and in the small particle sizes at least, are subject to electrostatic attraction which causes particles to adhere to the surfaces of the system. In particular the crud deposits on the fuel element cladding. The fuel elements generally have Zircalloy clad surfaces which are further coated with a thin layer of zirconia (ZrO.sub.2). The ZrO.sub.2 layer influences the deposition rate of the crud due to the natural attraction between the surface electrical charges of the crud and the ZrO.sub.2.
Particles suspended in solution acquire surface charges as a consequence of surface hydrolysis reactions and adsorption of ions from the solution. Solution pH most generally controls the polarity and extent of the charge for oxide surfaces. As the particle dimensions become smaller, the surface charge becomes more and more important in determining the properties of the suspended particles. When the particle dimension decreases to between 10 and 100 .ANG., the suspended solid is referred to as a colloidal particle, and the surface charge becomes the most significant element in determining properties. The stability of suspended particles with respect to flocculation and coagulation or precipitation is determined by the surface charge. The particles move when placed in an electric field and this process is referred to as electrophoresis. The charged particles, as a consequence of their mobility in an applied electric field, may be removed from solution and collected or deposited on an electrode surface. This electrophoretic deposition process is the basis for a significant technology in the industrial coating field, e.g., in depositing thin films of insulating materials, polymer and paint coatings. The electrostatic forces can cause charged particles to adhere to surfaces.
A procedure advanced for the removal of crud from nuclear reactor coolant streams by taking advantage of the magnetic properties of crud is set forth in U.S. Pat. No. 4,594,215, which issued on June 10, 1986 to the present applicants. The magnetic filter of the '215 patent is a successful crud collector; however, the dangers inherent in radiation exposure and the economics of nuclear reactors tend to encourage further searching for still more efficient procedures. The present invention provides such an improved system.